The present invention relates to a wafer of a compound semiconductor single crystal having an epitaxial layer of gallium phosphide, gallium arsenide or a mixed crystal of gallium phosphide arsenide from which light-emitting diodes capable of emitting very intense electroluminescence can be manufactured in a greatly improved yield of acceptable products by virtue of the substantial absence of surface defects.
It is a well established technology that light-emitting diodes in the green to red wavelength region are manufactured from a wafer of a compound semiconductor single crystal such as gallium phosphide GaP and gallium arsenide GaAs having an epitaxial layer of a mixed crystal of gallium phosphide arsenide of the formula GaAs.sub.1-x P.sub.x, in which x is zero or a positive number not exceeding 1. The epitaxial layer is formed usually by the method of vapor-phase epitaxial growth due to the easiness in the control of the mixing ratio of the components in the mixed crystal. The single-crystalline substrate wafer used therefor usually has a crystallographic orientation that the surface thereof is inclined by one to several degrees from the {001} plane in the direction of &lt;110&gt;.
One of the problems in the epitaxial growth on the surface of a single crystal wafer in the above mentioned crystallographic orientation is that the surface of the epitaxial layer sometimes has crystallographic defects called "hillocks". The surface defects of this type are mainly responsible for the decreased yield of acceptable products in the manufacture of lamps and display units in which light-emitting diodes prepared from the epitaxial wafer are utilized. In addition, it is a trend in recent years that light-emitting diodes emitting more and more intense electroluminescence are required since a single light-emitting diode has been used practically in the lamp and display uses. In this regard, the epitaxial wafers of a compound semiconductor of the above mentioned prior art are far from satisfaction of the demands.